The efficient use of exhaust gas recirculation (EGR) is very important to all modern internal combustion engines, including both gasoline and diesel engines. Efficient use of EGR generally supports the objectives of realizing high power output from these engines while also achieving high fuel efficiency and economy and achieving increasingly stringent engine emission requirements. The use of forced-induction, particularly including turbochargers, in these engines is also frequently employed to increase the engine intake mass airflow and the power output of the engine. However, turbochargers are also powered by exhaust gas, so the efficient use of EGR and turbocharged forced-induction necessitates synergistic design of these systems.
Turbocharged Diesel engines must be particularly efficient in the use of the energy available in the exhaust system, particularly EGR gas flows, in order to improve overall engine efficiency and fuel economy. Diesel EGR systems are required to deliver high volumes of EGR to the intake air system of the engine. In order to do so, the exhaust system must provide enough pressure change through the system, including the flow control valve, bypass valve and cooler to drive the desired EGR flow into the boosted intake system. The exhaust system must also provide adequate energy so that the turbine has sufficient power to provide the desired boost. Typical Diesel engine EGR systems feed EGR passages off various exhaust system components. EGR feed passages off the turbine housing have been proposed; however, such EGR feed passages have generally been at less than optimal angles to the desired gas flow direction within the turbine volute, through the use of elbows and the like, thereby creating high flow losses and low efficiency, thereby reducing the amount of EGR flow available for use in the air intake system. Such arrangements do not provide a sufficient volume of intake EGR.
In U.S. Pat. No. 6,430,929, a design has been proposed to associate an EGR outlet with a turbine volute and EGR valve. This design locates the EGR outlet tangentially to the volute and substantially linearly along the flowstream entering the turbine housing inlet. Thus, the EGR outlet is located at the volute inlet and the EGR outlet appears to define the volute inlet. The turbocharger described in this patent incorporates an EGR valve having a flanged elbow, where the hole pattern on the flange can be adjusted to orient the elbow to accommodate varying engine arrangements. The use of the elbow may also be necessitated by the in-line or linear arrangement of the EGR outlet and turbine inlet. However, use of the elbow configuration has an efficiency loss associated therewith. The turbocharger of the '929 patent also incorporates a variable geometry nozzle that is used to increase back pressure in the EGR system. While potentially useful, the costs of variable nozzle turbochargers are significantly higher than those having fixed nozzles. Further, increases in back pressure observed by closing the turbine vanes of a variable nozzle are nearly outweighed by the resultant increase in boost pressure of the intake air, such that the desired increases in EGR flow in the induction system are not achievable.
Accordingly, it is desirable to provide turbine housings, turbochargers and intake air systems that use them and associated methods of use that enhance EGR available for use in the induction system while at the same time providing sufficient exhaust gas flow to drive the turbine and generate the desired pressure boost and air induction into the air intake system, regardless of whether the turbochargers use either fixed or variable nozzle turbines.